Date: Thu, 18 Oct 2012 19:42:18 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: John Baldwin <jhb@freebsd.org> Cc: freebsd-hackers@freebsd.org, Tristan Verniquet <tris_vern@hotmail.com> Subject: Re: syncing large mmaped files Message-ID: <20121018164218.GR35915@deviant.kiev.zoral.com.ua> In-Reply-To: <201210180939.34861.jhb@freebsd.org> References: <SNT124-W20F26CF7B468F7F09B9B4983760@phx.gbl> <20121018083537.GQ35915@deviant.kiev.zoral.com.ua> <201210180939.34861.jhb@freebsd.org>
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--DI3e56nQDAJ1LWZd Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Thu, Oct 18, 2012 at 09:39:34AM -0400, John Baldwin wrote: > On Thursday, October 18, 2012 4:35:37 am Konstantin Belousov wrote: > > On Thu, Oct 18, 2012 at 10:08:22AM +1000, Tristan Verniquet wrote: > > >=20 > > > I want to work with large (1-10G) files in memory but eventually sync > > > them back out to disk. The problem is that the sync process appears to > > > lock the file in kernel for the duration of the sync, which can run > > > into minutes. This prevents other processes from reading from the file > > > (unless they already have it mapped) for this whole time. Is there > > > any way to prevent this? I think I read in a post somewhere about > > > openbsd implementing partial-writes when it hits a file with lots of > > > dirty pages in order to prevent this. Is there anything available for > > > FreeBSD or is there another way around it? > > > > > No, currently the vnode lock is held exclusive for the whole duration > > of the msync(2) syscall or its analog from the syncer. > >=20 > > Making a change to periodically drop the vnode lock in > > vm_object_page_clean() might be possible, but requires the benchmarking > > to make sure that we do not pessimize the common case. Also, this opens > > a possibility for the vnode reclamation meantime. >=20 > You can simulate this in userland by breaking up your msync() into multip= le > msync() calls where each call just syncs a portion of the file. Be aware that this is much-much slower than msyncing the whole file, even if file is very large. The reason is that pager initiates asynchronous _immediate_ clustered write for such situations. Async writes (AKA bdwrite()) are only specified for full range msyncing. >=20 > > Anyway, note that you cannot 'work with large files in memory', even if > > you have enough RAM and no pressure to hold all the file pages resident. > > The syncer will do a writeback periodically regardless of the applicati= on > > calling msync(2) or not, with the interval of approximately 30 seconds. >=20 > You can mmap with MAP_NOSYNC to prevent the syncer from writing the file = out > every 30 seconds. This also prevents msync(2) from syncing the region. The flag is fine for throw-away data, but not for the scenario that was described, I think. --DI3e56nQDAJ1LWZd Content-Type: application/pgp-signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (FreeBSD) iEYEARECAAYFAlCAMWoACgkQC3+MBN1Mb4iV7ACfeO+DqO2Onc8uMS29tjTbykJF Ek0An1i+6oS2OaxLly9sI5pAGmKlXw8F =UG69 -----END PGP SIGNATURE----- --DI3e56nQDAJ1LWZd--
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